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Nanoantennas could bring sensitive detectors, optical circuits

22.08.2002

Researchers have shown how tiny wires and metallic spheres might be arranged in various shapes to form "nanoantennas" that dramatically increase the precision of medical diagnostic imaging and devices that detect chemical and biological warfare agents.

Engineers from Purdue University have demonstrated through mathematical simulations that nanometer-scale antennas with certain geometric shapes should be able to make possible new sensors capable of detecting a single molecule of a chemical or biological agent. Such an innovation could result in detectors that are, in some cases, millions of times more sensitive than current technology.

The nanoantennas in the simulations are made of metal wires and spheres only about 10 nanometers thick – or roughly 100 atoms wide. They are an example of "left-handed" materials, meaning they are able to reverse the normal behavior of visible light and other forms of electromagnetic radiation.

Ordinary materials, such as glass, plastic, air and water, are called "right-handed" because of the way in which light bends as it penetrates a material. Left-handed materials have the ability to bend waves of electromagnetic radiation in the opposite direction of right-handed materials. This unusual property means that such materials might be used to create a so-called "super lens" that drastically improves the quality of medical diagnostic images.

The Purdue researchers are the first to show precisely how left-handed materials – the nanoantennas – could be applied to visible light and other electromagnetic radiation consisting of short wavelengths. Scientists at the University of California at San Diego proved two years ago that left-handed materials could be applied to devices that use microwaves, which are much longer than the waves needed for medical imaging, and for sensors used in spectroscopy to detect chemicals and biological agents. The phenomenon was first predicted in the late 1960s.

"All of the work in this area so far has been done in the microwave spectral range," said Vladimir Shalaev, a professor in Purdues School of Electrical and Computer Engineering. "We believe that this is the first project for how these types of materials can be used in the visible range of the electromagnetic spectrum."

The Purdue researchers have shown in theory how the same phenomenon could be scaled down to devices only nanometers wide. The research also shows how nanoantennas with specific shapes are critical for receiving certain frequencies of electromagnetic radiation. The findings were published in the March issue of the Journal of Nonlinear Optical Physics and Materials. The paper was written by Shalaev, Viktor A. Podolskiy, a post-doctoral fellow at Princeton University, and Andrey K. Sarychev, a senior research scientist at Purdue.

Purdue researchers plan to take the work a step further by creating the nanoantennas and conducting experiments to support the theoretical calculations, Shalaev said.

"Left-handed materials might have loads of applications," Shalaev said. "We dont know yet the full potential of these materials because its a really new field."

The researchers showed how the nanoantennas could be created by arranging pairs of tiny wires parallel to each other. That arrangement, in theory, enables the nanoantennas to achieve a "negative index of refraction," said Shalaev, a physicist by training.

Light and other forms of radiation bend as they pass through a material. Physicists measure this bending of radiation by its "index of refraction." The larger a materials index, the slower light travels through it, and the more it bends, or changes direction when going from one material to a different one. Because left-handed materials bend light in precisely the opposite direction as right-handed materials, they are said to have a "negative index of refraction."

"With these new types of materials, it may be possible to accomplish better performance than all existing materials, in terms of making images and manipulating light," Shalaev said.

The nanoantennas work by using clouds of electrons, all moving in unison as if they were a single object instead of millions of individual electrons. These groups of electrons are known collectively as "plasmons."

Researchers hope to one day use nanoantennas to create more compact, faster circuits and computers that use packets of light, called photons, instead of electrons for carrying signals. Photons travel much faster than electrons, but, unlike electrons, they do not possess an electrical charge. This lack of an electrical charge makes it far more difficult to manipulate photons.

"Because electrons are negatively charged particles, its easy to manipulate them," Shalaev said. "You just apply a field and they start moving.

"It turns out that, by employing these plasmonic nanomaterials, you should be able to manipulate light. You can guide light. You can basically simulate all the basic fundamental properties of electronic circuits, but in this case photons start to work."

Such photonic circuits could usher in a new class of ultrasensitive sensors that detect tiny traces of chemicals and biological materials, making them useful for applications including analyzing a patients DNA for medical diagnostics, monitoring air quality for pollution control and detecting dangerous substances for homeland security.

"This could be a way to dramatically enhance sensitivity in detecting molecules," Shalaev said. "Thats a great goal. These plasmonic nanomaterials accumulate electromagnetic energy in extremely small areas, nanoscale areas. Its like focusing light in areas much smaller than the wavelengths of light.

"Conventional lenses cannot focus light in an area smaller than the wavelength of the light. When you use these plasmonic nanomaterials, which act like nanoantennas, you do focus light in areas much smaller than the wavelengths. This means that metallic nanostructures might be able to detect even a single molecule of a substance, which is not possible with conventional optics."

The nanoantenna shapes used in the simulations ranged from single spheres to more complex geometric configurations called "fractals," in which the same shape is repeated in smaller and smaller sections.

Using metallic structures only a few nanometers thick is critical to applying the technique to visible light.

"Light cannot go into metals," Shalaev said. "But when you take a very small piece of metal, the light goes through completely and you very efficiently excite the whole piece of metal."

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